oil-immersion objective lens
oil-immersion objective lensan objective lens used to examine objects at very high optical power (at least 1000x). When in use, the small gap between the lens and the object is filled with oil of a high refractive index, preferably identical with that of the lens itself. In these circumstances no refraction takes place until light leaves the objective lens. see MICROSCOPE.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005